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  • 101Hero Hacks: Control Board LEDs

    Posted on January 9th, 2017 admin No comments
    When I document.write(" Wiring, Cooling Fan" href="http://blog.graywriterrv.com/?p=1549">mounted the control box to a pylon, I moved the factory LED to the other side so it would be visible from where I sit. Since then, I've been annoyed that this LED is also on when the USB cable is plugged in, so I can't easily tell at a glance if I've remembered to turn off the printer. I have to come up and check the switch, what a chore! Or unplug the USB cable, even worse!
    The first hack -- move the LED from bottom right to top left in the image

    The first hack -- move the LED from bottom right to top left in the image

    Adding a Power LED - First Try

    I figured the factory LED was run off 5V -- there are two 7805 voltage regulators on the board and the LED appears to come right off one of them -- and that I could just put another LED across 12V off the power switch to ground. And that worked fine -- turn on the switch and the new LED lights up. There's my power-on/off indicator.

     
    To my surprise and annoyance, that LED also lights when just the USB cable is plugged in -- just like the existing LED.
     
    I would have thought the 12V power supply would be totally separate from the 5V USB and UART. As far as I can tell (can hardly see the traces, let alone track them), there's only a 1000u cap across the inputs before the voltage regulators.

    Second Try -- Using 5V from the USB Cable

    The USB jack also has 5V at pin 4 supplied by the hero board, rather than from the computer. So much for an indicator across pins 1 and 4 there.  It's always on when the power's on.  It's always on when the USB cable is connected.  It's the same as the existing factory LED.

    A Standby Indicator LED

     For now, I've put a small green LED from ground to the unused third pin of the switch. It glows when the printer is OFF, a kind of "safe" or "standby" light.   It's drilled through the top of the case and is readily visible.    I used a 470 ohm resistor but in retrospect a 1K resistor would have been fine -- the LED doesn't have to light up my office at night.
    Location of the standby LED, held in place by the resistor

    Location of the standby LED, held in place by the resistor

    Both LEDs

    Both LEDs in place

    Switch off, standby LED on

    Switch off, standby LED on

    Conclusion

    Not quite what I'd wanted, but it met my goal of indicating that the printer/board is powered down.
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    Further Reading

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